The orphan nuclear receptor ROR?t has a central role in the differentiation of Th17 cells, a subset of T helper cells that secrete the inflammatory cytokines IL-17, IL-17F, and IL-22. Th17 cells have recently been shown to have major pathogenic functions in a variety of murine autoimmune disease models and in graft versus host disease, and they have also been implicated in human autoimmune disease and asthma. ROR?t is induced in antigen-stimulated na[unreadable]ve T helper cells in the presence of TGF-??combined with IL-6, IL-21, or IL-23, and thereafter directs the expression of the Th17 lineage cytokines. We have shown that mice defective for expression of ROR?t lack Th17 cells and are resistant to autoimmune disease. A better understanding of how ROR?t functions in inducing the Th17 lineage is therefore likely to provide important information for developing new therapeutic approaches for inflammatory diseases. To study the mechanism of action of ROR?t, we have developed an insect cell-based assay to identify host factors involved in its transcriptional activity and to screen for small molecules in high throughput assays. Cells in which a Gal4-ROR?t fusion protein directs expression of luciferase under regulation of the Gal4 promoter have been used in genome-wide RNAi and small molecule compound screens to identify inhibitory or enhancing activities. The screening assay has been optimized for a 384-well format, and multiple candidate genes have been identified in the RNAi screen. A proof-of-principle screen with a limited small molecule compound library has also been successfully performed. We propose to collaborate with the NIH Molecular Libraries Screening Network to perform a high throughput screen to identify compounds that modulate the activity of ROR?t. Compounds that affect transcriptional activity in the primary screen will then be examined for activity in T cell differentiation assays and other secondary screens to determine if they act directly on ROR?t or if they affect other components of ROR?t-directed transcription. Compounds that specifically affect Th17 cell differentiation will be of great utility in dissecting the transcriptional program of these inflammatory T cells and will guide us in developing new therapeutic approaches for autoimmune disease and graft versus host disease. [unreadable] [unreadable] [unreadable]